[Jacob-list] DEF:Freckling, Ticking, Mottling

[Grose]

Fred and all,

I have a problem. early on in our Jacob career [15 years ago], we bred 2 Jacob rams from 2 inbred bloodlines to Corriedale ewes [white and colored] for handspinners fleeces. Ram A produced all black lambs. Ram B produced all marked lambs, which ranged from white blazed faces to typical Jacob markings. We have seen similar results in our occasional CotswaldXFinn sheep. A third bloodline ram produced a few Lambs with dark spots on a lighter grey rear-end. In addition, we see more "freckling" of all persuasions in the more outcrossed lambs.

Here are a few opinions and observations:
1] There are variations in the piebald gene among the bloodlines of Jacob sheep.Strong opinion
2] Some Jacob lines carry a dominant piebald allele. Strong opinion
3] All freckling does not have the same genetic origin. Mild opinion
4] Freckes and ticking and mottling may have similar genetic origins. Mild opinion
5] Freckling, etc. has a huge environmental component. Strong opinion
6] Some freckling may be caused by lack of "alignment" between the piebald genes. Mild opinion
7] Freckling, etc. is influenced by a bunch of genes. Vague opinion

It pays to remember that very few traits accommodate the Simple Mendelian Inheritance model. Most are a result of codominant genes or are influenced by many genes. In these cases it is practically impossible to eliminate these traits from the genotype. We just have to do the best that is reasonable and not fall into the "throw the baby out with the bathwater" syndrome.

Neal Grose
  ----- Original Message ----- 
  From: Jacobflock at aol.com 
  To: jacob-list at jacobsheep.com 
  Sent: Sunday, April 07, 2002 3:36 PM
  Subject: [Jacob-list] DEF:Freckling, Ticking, Mottling


  There is confusion among these terms and by using the terms adopted by color geneticists I guess I added to the confusion, i.e., ticking and freckling  

  In 1988 (?), the Committee for Genentic Nomenclature for Sheep and Goats agreed on a set of terms, loci and alleles and expressions for sheep and these were adopted to reduce confusion and provide a common reference for loci, alleles and their effects.

  The COGNOSAG terms related to the Spotting locus and freckling and ticking:
  Freckles describe small melanocytes with pigmented fiber present at birth, not to be confused with the larger Jacob piebald pattern.  Some "freckles" may appear through excitement of the melanocyte by the sun.  Hence, freckles on the back (along the spine) by be induced by ultra violet rays and therefore, might be differentiated from "freckles" on the side ... for pigmented skin with non-pigmented fiber see skin mottling further on.  

  Ticking looks like  an onslaught of freckles but is the appearance, over time, of melanocytes that were not present in the birth coat.  The ticking shows about the size of a dull pencil point, may spread to a density such that it presents, at the extreme, the appearance of a completely pigmented fleece but is actually hundreds of "ticks".    This is most often observed in successive shearings and the fleece may look like a black and gray mix.  It is widely known that the melanin must come from melanocytes near the papilla and the melanin must get moved to the fiber itself.  How the melanocyctes grow in a fashion that does not seem to be in a contiguous area is a question.  Ticking, until 1990(?), was called pigmented multiple spotting (PIMS). Since the Jacob is a full (dark) color ED, dominant black  BB and recessive piebald ss, the piebald activity must in some fashion be compromised such that the piebald allele allows ticking.  Think of Ticking as a "time bomb".

  Skin mottling, (also a function of the Spotting locus) is the presence of melanin in the epidermal layer, but since there are no papilla melanocyte structures in the sub dermal area, migration of the melanin to the fiber can't, by definition, occur.  Hence, you see darker skin and white fiber.  In the Jacob, skin mottling may be the expression of piebald action that subdues the sub dermal melanocyte (expresses white fleece) but not the epidermis (the darker skin color).   Skin mottling (presence of melanin) can also be excited by the sun since "black/eumelanin" is a protective color .. look for this along the back/spine as well.

  What does dominant mean when referring to freckles and ticking?  You see dominant expressions.  When dominant traits are observed ... it is not a matter of degree (the more you see the more dominant, or, its only a little, therefore not dominant).   A dominant gene, paired with its allele, simply covers up the phenotypic expression of that gene (whether homozygous or heterozygous).   If traits seem to be sex linked or sex related (freckles in maternal lines) a body of evidence can be built but would have to be done by all progeny perhaps outside the registration process (which seems to be 'selective').

  It is my OPINION, absent evidence to the contrary and further understanding of the Spotting locus and epistasis, that indiscriminate breeding of freckled, ticked and early grayers (noted by COGNOSAG) will change the basic genetic soup of the Jacob from ED/ED B/B ss and White (Alderson, 1979), to something like Freckle or Gray or  Ticked  ...  and the clear black and white may be gone.  The problem and questions also exist in the JSS in England.

  There is a considerable body of evidence related to the Jacob and other piebald sheep during the period, say 1925 to 1990 but most work has been related to other species since 1990.  Hence my excitement over Gary Anderson's attempts with identical twin Jacobs this past Fall which could lead to avenues to understand color/piebald, breed specific markings/random markings, and the 2 and multiple Horn locus.  (About a year ago, UC Davis made an interesting finding regarding the piebald locus in the Border Collie (?)  ... the piebald mechanism doesn't fit either of the two mechanisms that are generally accepted.)   

  Fred Horak




-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.pairlist.net/pipermail/jacob-list/attachments/20020409/a3e13352/attachment.htm